1. Field of the Invention
This invention relates to an organic electroluminescence (hereinafter referred to as EL) display panel with a plurality of organic EL elements formed in a prescribed pattern on a substrate. Each of the EL elements is composed of at least one thin film (hereinafter referred to as an organic functional layer) containing a light emitting layer of an organic compound material which exhibits EL emitting light due to current injection.
2. Description of the Related Art
The organic EL element includes an anode transparent electrode, an organic functional layer and a cathode metallic layer which are successively stacked on a transparent substrate. The organic functional layer may be e.g. a single layer of a light emitting layer; a double layer structure composed of an organic hole transporting layer and a light emitting layer; a triple layer structure composed of an organic hole transporting layer, a light emitting layer and an organic layer electron transporting layer; or a laminated structure with an electron- or hole-injected layer inserted between these suitable layers.
The organic EL display panel with a plurality of organic EL elements formed in a prescribed pattern on a substrate can constitute a matrix display panel. The matrix display panel, as disclosed in JP-A-8-315981, canprovide a full-color display. The full-color display is a light emitting device having an image display arrangement consisting of a plurality of light-emitting pixels of organic EL elements arranged in rows and columns which are orthogonal to each other. For example, the matrix display type of display panel includes a plurality of EL elements, each including a row electrode containing a transparent electrode layer, an organic functional layer and a column electrode containing a metallic electrode layer orthogonal to the row electrode, which are successively stacked on a substrate, and a stripe or grid-like insulating film. The row electrodes, each being formed in a belt shape, are arranged in parallel to one another at prescribed intervals. The column electrodes are also arranged likewise. In this way, the matrix display type of display panel has an image display arrangement consisting of a plurality of light emitting pixels of organic EL elements located at crossing points of electrodes in a plurality of rows and columns.
Traditionally, the insulating film was made of a relatively transparent material such as polyimide, or material used in the black matrix of a liquid crystal display.
In the organic EL display panel, generally, the column electrode was made of a metallic material such as Al in view of a work function. However, since the metallic electrode layer of e.g. Al has a high reflectance to visible light, a part of external light incident on the organic EL display panel from the outside is reflected by the column electrode and is emitted from the substrate surface. A part of the external light is also reflected by the substrate itself and becomes reflected light. Therefore, the organic EL display panel itself has a high reflectance and hence could not provide the display with high contrast.
In order to solve the above problem, JP-A-7-142170 discloses an organic EL display panel whose contrast was improved by providing a polarized layer outside the row electrode. However, a polarized filter which is used as the polarized layer is not only expensive, but has high dependency of the reflectance on an angle. This presented a problem that the contrast largely varies according to the viewing angle.
The inventors of this invention have first found the relationship between the reflectance of an insulating film and an optical density abbreviated OD value thereof and succeeded in specifying the OD value which is indispensable to obtain a good reflectance for an organic EL display panel.
The invention has been accomplished to solve the above problem, and intends to provide an organic EL display panel with improved contrast.
In order to attain the above object, according to a first aspect of the invention, there is provided an organic electroluminescence (EL) display panel comprising a plurality of EL elements arranged through an insulating film, each of said EL elements including a first display electrode, at least one organic functional layer containing a light emitting layer of organic compound and a second display electrode which are successively stacked on a substrate, wherein said insulating film has an OD value of 0.5 or more in a visible light range.
According to a second aspect of the invention, in the organic EL display panel according to the first aspect, said insulating film has a current density of 10xe2x88x928A/mm2 or less when a voltage of 30 V is applied.
According to a third aspect of the invention, in the organic EL display panel according to the first aspect, said insulating film has a volume resistance of 1.0xc3x971012xc2x7xcexa9cm or more.
According to a fourth aspect of the invention, in the organic EL display panel according to any one of the first to third aspects, an outermost surface of said insulating film has a sloping surface which slopes at an angle smaller than a vertical angle to said substrate in an area oriented toward the surface of said substrate.
According to a fifth aspect of the invention, in the organic EL display panel according to any one of the first to fourth aspects, said insulating film is formed in a grid pattern in at least a part between said organic EL display elements.
According to a sixth aspect of the invention, in the organic EL display panel according to anyone of the first to fifth aspects, said insulating film has a partition on at least a portion of the outermost surface.
According to a seventh aspect of the invention, the organic EL display panel according to any one of the first to sixth aspects further comprises a lightproof layer on the surface of said substrate.
According to an eighth aspect of the invention, in the a organic EL display panel according to the seventh aspect, said lightproof layer is an ND filter.
According to a ninth aspect the invention, in the organic EL display panel according to anyone of the first to sixth aspects, said substrate is a lightproof substrate.
According to a tenth aspect of the invention, in the organic EL display panel according to the ninth aspect, said lightproof substrate is an ND filter.